1. Field of the Invention
The invention relates to a method of processing imaged printing plate precursors.
2. Description of the Related Art
Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-adhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Lithographic printing masters are generally obtained by the image-wise exposure and processing of a radiation sensitive layer on a lithographic support. Imaging and processing renders the so-called lithographic printing plate precursor into a printing plate or master. Image-wise exposure of the radiation sensitive coating to heat or light, typically by means of a digitally modulated exposure device such as a laser, triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer. Although some plate precursors are capable of producing a lithographic image immediately after exposure, the most popular lithographic plate precursors require wet processing since the exposure produces a difference in solubility or difference in rate of dissolution in a developer between the exposed and the non-exposed areas of the coating. In positive working lithographic plate precursors, the exposed areas of the coating dissolve in the developer while the non-exposed areas remain resistant to the developer. In negative working lithographic plate precursors, the non-exposed areas of the coating dissolve in the developer while the exposed areas remain resistant to the developer. Most lithographic plate precursors contain a hydrophobic coating on a hydrophilic support, so that the areas which remain resistant to the developer define the ink-accepting, hence printing areas of the plate while the hydrophilic support is revealed by the dissolution of the coating in the developer at the non-printing areas.
In order to be able to evaluate the resulting lithographic printing plates for image quality, such as image resolution and detail rendering (usually measured with an optical densitometer) before mounting them on the press, the lithographic printing plate precursors contain a colorant (dye or pigment) in the radiation-sensitive coating. Such colorants provide a contrast between the image and the hydrophilic support (where the coating has been removed). Besides allowing for the evaluation of the image quality, a high contrast between the image and the hydrophilic support is required to obtain a good image registration (alignment) of the different printing plates in multi-colour printing in order to ensure image sharpness (resolution) and a correct rendering of the colours in the images present.
It is a well known phenomenon that offset lithographic printing plate precursors containing a coating with sufficient colouration in order to generate printing plates that have image parts that are visible and measurable after processing, can leave significant contaminants of the colouration dye or pigment within the developer unit, the rinsing unit and the gumming unit. Dye residues turn the processing solutions, being the developer, rinsing solution or gum coloured and cannot be so easily removed. After processing an amount of lithographic printing plate precursors, the accumulation of the colorant in the processing solutions is leading to very dark coloured solutions, which ultimately may lead to premature replacement by the operator, while the activity of the processing solution, is still sufficient. This leads to an increased amount of waste at the customer site. In case the processing of the printing plate precursor is performed in one single step, a single processing solution is used to both develop the imaged printing plate precursor by removing predominantly the coating in the non-printing areas and also to provide a protective layer or gum coating, over the entire plate surface. In this respect, the non-printing areas tend to be coloured due to coloured gum being left on the plate surface. Additionally, contamination by coloured gum of the conveyor street and the belt to the punch bender can be observed. Also the plate stacker gets visually stained. Moreover, processor parts, which are in contact with the dark coloured processing solutions show accumulation of coloured residue after evaporation of the solvent of the processing solution. This all requires a high frequency of cleaning of the processing equipment.
The use of contrast-providing colorants obtained from leuco dyes that become coloured in the presence of a thermal acid generator, is described for example, in U.S. Pat. Nos. 7,402,374, 7,425,406, and 7,462,440. The colouring of the printing areas of negative working printing plate precursors is initiated by the image-wise exposure to obtain a good visibility of the image before performing an on-press development of the plate precursor. Hence the printing plates are all designed for on-press development without undergoing alkaline development in a processing apparatus. This solution however is not compatible with positive working lithographic printing plate precursors, since the exposed parts of the radiation sensitive coating are dissolved in the developer and still would give rise to a colouration of the developer and the rinsing solution. Moreover, the coating in printing areas is not coloured since it is not exposed by the laser and as a result a poor image contrast is obtained.
Thus, there is a need to provide colour image contrast in positive and negative working lithographic printing plates without causing problems related to the colouration of the processing solution due to the processing of the printing plate precursors.
A colour contrast image in imaged lithographic printing precursors can be obtained after processing by contacting the imaged precursor with a colouration solution containing a colourless form of a photochromic compound as disclosed in US 20120045720 A. Residual amounts of this compound attached to the oleophilic surface of the imaged precursor can be changed to its coloured form when exposed to UV light. This solution requires however a processor having an extra tank to contain the colouring solution which increases cost and complexity of the processor.
In US 20100316956 A a method is provided to improve contrast between the image and background of lithographic printing plates by applying a colouring fluid to the imaged and processed precursor. Also in this case, an additional processing solution, which must be coloured, is required during the plate making process.
EP 0419095 A and EP 0127477 A disclose radiation sensitive compositions containing dyes which undergo a colour change on baking after exposure and development. The colour change is to obtain an indication of whether or not the heating has been adequately done. All radiation sensitive compositions however do comprise a colorant which will lead to a strong colouration of the developer.
WO2010/101632A claims an imagable element having thereon a radiation-sensitive imagable layer comprising a pigment and a dye that can change colour when heated. This combination gives at the same time a high visual image contrast of the printing plate for visual inspection and an indication of a postbake step due to a decrease in colour density. The high visual contrast of the imagable layer will lead to strong colouration of the developer.
WO2006/005688A and EP1614541A describe a negative-working heat-sensitive lithographic printing plate precursor which is capable of providing directly after image-wise heating, a visible image. This so-called ‘print-out image’ is formed before the development of the printing plate precursor and enables the quality control of the printing plate precursor before the development step.
In EP 1865382 A a method for preparation of lithographic printing plates is disclosed which comprises an overall exposure to light after the image exposure and development step of the radiation sensitive element. The coverage of the photosensitive layer by an amount of developer that blocks the oxygen, preventing any inhibition effect on the polymerisation reaction during the overall exposure, leads to a good developing property and an excellent printing durability of the resulting printing plate.
US 2010/0310989 A describes a method of obtaining lithographic printing plates by contacting an infrared radiation-imaged negative-working lithographic printing plate precursor with a processing solution having a pH less than 9 and comprising a UV photoinitiator, followed by a flood-wise exposure with UV radiation to increase the imaging sensitivity and the press life of the resulting plate. The flood-wise exposure with UV radiation of the precursor in the presence of a UV photoinitiator is to be seen as a post-treatment in order to further polymerise the radiation sensitive layer, improving thereby the sensitivity of the precursor and the hardness of the coating in the image areas of the plate. No solution for reducing colouration of the processing solution is provided.
There is a need for an improved means for providing contrast between the image and background of lithographic printing plates, prepared from negative and positive working lithographic printing precursors without causing problems related to the colouration of the processing solutions.